Conventionally, in an imaging device such as a monitoring camera that continuously performs photographing night and day (hereinafter, simply referred to as imaging device), infrared light is detected and photographing is performed at night. A photodiode that is a light-receiving unit of an imaging sensor such as a CCD sensor or a CMOS sensor can receive light of up to a near-infrared wavelength band around 1300 nm. Thus, an imaging device using these imaging sensors can perform photographing of up to an infrared band in principle.
Note that a wavelength band of light with a high luminosity factor of a human is 400 nm to 700 nm. Thus, when near-infrared light is detected in an imaging sensor, redness is increased in a video seen with a human eye. Thus, in photographing in daytime or in a bright indoor place, it is preferable that an infrared cut filter to block light in an infrared band is provided in front of the imaging sensor and light with a wavelength being 700 nm or longer is removed in order to adjust sensitivity of the imaging sensor to a luminosity factor of a human. On the other hand, in photographing at night or in a dark place, it is necessary to perform photographing without providing the infrared cut filter.
As such an imaging device, an imaging device in which attachment/detachment of an infrared cut filter is performed manually or an imaging device in which an infrared cut filter is inserted/removed automatically is conventionally known. Moreover, in Patent Literature 1, an imaging device in which the above-described insertion/removal of an infrared cut filter is not necessary is disclosed.
Thus, an optical filter that has a transmission characteristic in a visible-light band, a blocking characteristic in a first wavelength band adjacent to a long-wavelength side of the visible-light band, and a transmission characteristic in a second wavelength band that is a part of the first wavelength band has been proposed (see, for example, Patent Literature 1). According to this filter, light can be transmitted in both of the visible-light band and the second wavelength band that is away from the visible-light band on the long-wavelength side, that is, on an infrared side of the visible-light band. For example, the second wavelength band overlaps with a wavelength band of infrared illumination and this filter is an optical filter that makes it possible to perform both of visible light photographing and infrared light photographing at night with infrared-light illumination. In the following, an optical filter that transmits light in a visible-light band and a second wavelength band on an infrared side and that blocks light in the other wavelength band in the above-described manner is referred to as a double bandpass filter (DBPF).